Sagi Nahor in Aramaic is a great light. The Talmud uses the expression to refer to a blind person, as in Rabbi Sheshet was a sagi nahor (Talmud Berachot 58a). In truth, Rabbi Sheshets eyes lacked light, and therefore sagi nahor is an ironic usage and a euphemistic way of referring to a disability.
One of the most famous sagi nahor was Rabbi Isaac Sagi Nahor (known in English as Rabbi Isaac the Blind) an early Kabbalist. Perhaps with Rabbi Isaac in mind, there are those who explain sagi nahor as someone whose eyes lack light to see the concrete world but are endowed with great light to see the spiritual world.
When referring to something shameful, the Bible and the Talmud have on occasion used the opposite expression to avoid speaking coarsely. Hence, Jobs wife tells him literally Bless (Barech) God and die (Job 2:9). This verse is translated as: Blaspheme God and die. So distressing is the notion of committing such a transgression that the Bible prefers to speak in opposites.
It was not until the 19th century that this form of ironic speech became known as leshon (the idiom of) sagi nahor. The great Yiddish writer Mendele Mokher Sforim wrote, And each one calls his friend, in the style of sagi nahor, a great intellect, meaning a world-class idiot. The biblical scholar Rabbi David Zvi Hoffman also refers to leshon sagi nahor as a rabbinic linguistic style.
From a talmudic expression for visual impairment to the contemporary term for euphemism, leshon sagi nahor is one of those phrases whose literal meaning will not help you very much without a brief foray into its history. It is therefore a window into the ancient roots of modern Hebrew.